Demand for strong, lightweight aircraft has led to widespread fabrication of aircraft parts from composite materials. Composite structures typically include inner and outer composite skins, with a core (e.g., a honeycomb core material) or stiffening member therebetween. These structures are commonly bonded via multiple curings in an autoclave. For example, the inner and outer skins may be separately pre-cured in an autoclave, then assembled with adhesive, a honeycomb aluminum or non-metallic core and uncured closure plies into a sandwich panel. The sandwich panel is cured once to cure the closure plies, and then vacuum bagged to a composite bond jig and again cured in an autoclave.
Co-curing methods have been introduced in an attempt to reduce the steps, man hours and expense involved in the above process. In co-curing, composite skins laid-up with adhesive and a honeycomb core are cured in a single cycle in the autoclave. However, strength is lost in co-curing due to dimpling of the composite plies inward, with nothing but the cell walls to compact the composite skins. Additional plies may be added, but they add weight and cost to the finished part. Additional problems inherent to honeycomb core elements are described in U.S. Pat. No. 5,604,010 to Hartz et al., and U.S. Pat. Nos. 6,632,502 and 6,458,309, both issued to Allen et al.
The above-mentioned Allen patents discuss use of an internal air bag to counterbalance autoclave pressure during a single stage curing method. An internal air bag is placed inside a hollow, open-ended, stiffened graphite fabric mandrel that becomes an integral part of a finished aerostructure article. The lay-up (i.e., mandrel, bag, uncured composite layers and plies) is cured in an autoclave. Similarly, U.S. Patent Application Publication No. 2006/0006599 by Shahidi et al. describes a device that fits to a mould tool where an inflatable body with a pressurizable seal exits from the mould tool. When the body is inflated, the device prevents distortion and excess inflation of the pressurizable seal so that vacuum conditions can be applied there around.
As an alternative to inflatable mandrels, removable, rigid mandrels may be inserted within a prepreg lay up prior to autoclaving. The mandrels support the layup under autoclave pressures, and are removed from the final, cured product. For example, U.S. Pat. No. 6,589,472, issued to Benson et al., describes a combination tool/vacuum bag for compressing inner surfaces of composite parts. Composite parts, joints and webs are laid up around the tool/vacuum bag, and a separate, external vacuum bag is sealed to the assembly. Under vacuum pressure, the combination tool/vacuum bag expands outward to compress inner surfaces of the composite parts. After autoclaving, the assembly is reheated to soften and collapse the combination tool/vacuum bag, so that it can be removed.
Other techniques used in vacuum-bagging/bonding processes include placement of cushioning material between a layup and a vacuum bag, and using a vacuum bag, sealed to an edge of a component, in resin transfer molding. For example, U.S. Patent Application Publication No. 2005/0183818 by Zenker et al. suggests placement of an elastomeric caul beneath vacuum bagging material that is taped to a lay-up mandrel. The caul is stretched as negative pressure is applied, to draw the caul tightly against a composite component. Placing the caul between the component and the bagging material reduces wrinkling and deformation of the composite component by the bagging material. U.S. Patent Application Publication No. 2006/0049552 by Fish et al. describes vacuum assisted resin transfer molding for making a bond line between components. A vacuum bag is sealed against the edges of a component being bonded, and a vacuum is applied to draw injected resin through a flow path between the components.